In general, an electroluminescent element includes an inorganic electroluminescent element using an inorganic compound in a light-emitting element and an organic electroluminescent element using an organic compound in a light-emitting element. In recent years, active studies on practical realization of the organic electroluminescent element have been made because the element can achieve light emission at a low voltage and at a high luminance.
A basic structure of the organic electroluminescent element is obtained by forming a hole-injecting layer and an organic thin layer such as a light-emitting layer on a glass plate deposited with a thin film of an anode material such as indium-tin oxide (ITO) and further forming a thin film of a cathode material thereon, and there is known an element obtained by appropriately providing a hole-transporting layer or an electron-transporting layer on the basic structure. A construction of layers in the organic electroluminescent element is, for example, anode/hole-injecting layer/light-emitting layer/electron-transporting layer/cathode or anode/hole-injecting layer/hole-transporting layer/light-emitting layer/electron-transporting layer/cathode.
In recent years, it has been found that when a charge-transporting layers such as the hole-injecting layer and the hole-transporting layer are integrated between the light-emitting layer and the anode, the layers improve property of injecting holes into the light-emitting layer and serve as buffer layers which optimize a charge balance to significantly improve luminous efficiency and life of the element.
Hole-transporting materials used in the hole-transporting layer of the organic electroluminescent element are broadly classified into a low-molecular-weight hole-transporting material and a high-molecular-weight hole-transporting material.
As a method of forming the low-molecular-weight hole-transporting material into a film serving as the hole-transporting layer, a vacuum deposition method is mainly used and has the following characteristics. According to the method, it is easy to produce a multilayer using various materials having different functions, which allows a high-performance organic electroluminescent element to be formed. However, there is a problem in that it is difficult to control a thickness uniformly and to apply different materials depending on parts for achieving a large-screen and high-definition panel, and a large-size vacuum apparatus is required, resulting in an increase in production cost.
Further, as the method of forming the low-molecular-weight hole-transporting material into a film serving as the hole-transporting layer, a film formation method involving application of a solution containing the low-molecular-weight hole-transporting material has been studied toward practical use. However, it is necessary to improve this technique for practical use because segregation and phase separation due to crystallization of the low-molecular-weight compound are observed.
On the other hand, as a method of forming the high-molecular-weight hole-transporting material into a film, a solution application method such as a spin coating method, a printing method, or an ink-jet method is used because most of the material cannot be deposited by the vacuum deposition method. This method can easily enlarge a panel screen and is excellent in terms of mass production. However, this method has problems in that it is difficult to laminate coated films, and contamination with impurities is liable to occur. Therefore, an element including the high-molecular-weight hole-transporting material is inferior in element performance such as element efficiency or life as compared with the low-molecular-weight hole-transporting material. Therefore, there has been required a high-molecular-weight hole-transporting material having excellent hole-transporting performance and good film forming property.
As attempts for expressing the required properties, for example, Non Patent Literature 1 discloses polyvinylcarbazole and polysilane, and Patent Literature 2 and Non Patent Literature 2 report polymers having structures in which vinyltriphenylamine and triphenylamine are linked via methylene. However, organic electroluminescent elements including the compounds cannot improve the problems sufficiently because the luminous efficiency and stability of the elements are poor.
In addition, as a technique for enhancing the luminous efficiency of the organic electroluminescent element, a polymer material having a main chain of a n-conjugated polymer including an indolocarbazole unit integrated thereinto, and a light-emitting element including the polymer material have been disclosed. That is, Patent Literature 3 discloses a conjugated polymer obtained by bonding an indolocarbazole at a peripheral position, and Patent Literature 4 discloses a conjugated polymer obtained by introducing an indolocarbazole unit into a polyarylene main chain. However, each of the polymers is a n-conjugated polymer in which an indolocarbazole skeleton is bonded at a peripheral position. Although each of the polymers improves charge mobility, its HOMO energy level is shallow, and hence a difference between the level and the energy level of a host material is large. As a result, hole-injecting property deteriorates. Accordingly, it cannot be said that luminous efficiency has been sufficiently improved.